Sensors are used for sampling a fluid, mixing the fluid with a reagent, and making an analysis of the mixed sample. One form of sensor includes a capillary channel between two optical paths and a reagent in the channel. Another sensor is injection molded with a capillary channel between a base and lid. Electrodes and a reagent are located in the channel. A test fluid is drawn into the channel by capillary action and reacts with the reagent. In the first sensor, a light source is applied to one of the optical paths, and light from the light source is transmitted through the fluid in the channel and directed to a detector applied to the other optical path. In the second sensor an electric current acrossed the electrodes is measured.
A significant problem in the fabrication of optical reagent sensors is the production of a precision optical path length. This has been achieved by producing an optically clear part having a cavity of a certain depth. The cavity is covered by an optically clear lid. The precise depth of the cavity is difficult to produce repeatedly, but even if the depth can be repeatedly produced, it is very difficult to attach a lid and control the path length due to the tolerances of the method of attachment. In addition, if the attachment uses adhesives, variations in adhesives adds to the tolerances. Other types of attachment such as sonic welding each has their own variability.
The difficulty in repeatedly producing a cavity of a precise depth is critical when producing an optical reagent sensor that is to be used in the transmission mode. In the transmission mode the path length in the capillary gap is directly proportional to an analyte being tested. If an identical analyte is measured in two sensors that have different path lengths, the results reported will be different due to the path lengths. There is a need for a method of making optical reagent sensors with precise capillary channels that can be reproduced without variances or tolerances.
Similarly, the formation of a capillary channel in an injection molded sensor is difficult to produce. The usual way to fabricate sensors of this type is to precision screen print active areas within a capillary area formed by a shaped top lid. It is desirable to reduce the cost and assembly required by this construction. Moreover, it is difficult to provide electrodes in a small molded capillary channel of less than 0.005 inch in height and it is desirable to provide a sensor of this size with molded electrodes so that such a sensor could be used for electrochemical analysis.